1. Field
The present embodiment relates to a control circuit of power supply unit, power supply unit and control method of the same.
2. Description of the Related Art
As disclosed in Japanese unexamined patent publication No. 10(1998)-322931, there has been known a charging circuit capable of supplying electric power to a load circuit and a secondary battery while preventing electric power outputted from an external power supply (AC/DC converter) from exceeding its limit value. This charging circuit includes an input voltage detecting portion, a charging current control portion, and a charging current generating portion. The input voltage detecting portion detects an output voltage of the external power supply. The charging current control portion controls the charging current generating portion corresponding to a voltage detected by the input voltage detecting portion and the charging current generating portion changes the charging current of the secondary battery as described later.
For example, if the input voltage detecting portion detects a maximum output voltage value of the external power supply, the charging current generating portion sets a maximum value of charging current of the secondary battery and if the input voltage detecting portion detects a value between the maximum output voltage value and minimum output voltage value of the external power supply, the charging current generating portion reduces the charging current of the secondary battery. Further, when the input voltage detecting portion detects a minimum output voltage value of the external power supply, the charging current generating portion reduces the charging current of the secondary battery to zero. Then, in the charging circuit disclosed in the Japanese unexamined patent publication No. 10(1998)-322931, as described above, the charging current of the secondary battery is changed corresponding to a voltage value of the external power supply detected by the input voltage detecting portion, so that the output voltage of the external power supply is prevented from falling below the minimum output voltage by increasing/decreasing the charging current of the secondary battery so as to prevent an electric power outputted by the external power supply from exceeding its limit value.
On the other hand, as disclosed in Japanese unexamined patent publication No. 8(1996)-182219, there has been known an electronic apparatus capable of charging the secondary battery with an allowable maximum current of the secondary battery and external power supply in order to charge the secondary battery at the highest speed when corresponding machine is operated. This electronic apparatus includes first to fourth detecting means, a control means and a charging circuit.
The first detecting means detects a difference value between the maximum allowable charging current of the secondary battery and the charging current flowing into the secondary battery and the second detecting means detects a difference value (maximum available current) between the maximum allowable output current of the external power supply, and consumption current of machine. The third detecting means detects a difference value between the maximum available current and the charging current flowing into the secondary battery and the fourth detecting means detects a difference value between the maximum allowable applied voltage and a voltage applied to the secondary battery.
The difference values detected by the first detecting means, the third detecting means and the fourth detecting means are inputted to the control means. The control means determines whether or not the maximum allowable value or the maximum available value is exceeded based on the inputted difference value. If the control means determines that the maximum allowable value or the maximum available value is exceeded, it controls the charging circuit so that the charging circuit reduces the charging current flowing into the secondary battery so as to turn the difference value which is a basis for determining that the maximum allowable value or the maximum available current is exceeded to zero.
Contrary to this, if the control means determines that the maximum allowable value or the maximum available value is not exceeded, it specifies a difference value nearest zero and then, the charging circuit increases the charging current flowing into the secondary battery so as to turn the specified difference value to zero. Then, in the electronic apparatus disclosed in the Japanese unexamined patent publication No. 8(1996)-182219, if the aforementioned difference value is turned to zero, the charging current of the secondary battery maintains a state below the maximum allowable charging current of the secondary battery and consequently, when the electronic apparatus is actuated, the secondary battery can be charged at the highest speed in conditions in which the charging current of the secondary battery is near the maximum allowable current of the secondary battery.